1. Field of the Invention
This invention relates to a press-contact type semiconductor device which is used with semiconductor elements sandwiched between electrodes to which external force is applied, and more particularly, to a semiconductor device such as a mesa transistor, a gate turn-off thyristor, and a high-speed thyristor in which main and control electrodes are interlaced with each other.
2. Description of the Related Art
Because it possesses a high electrical conductivity, aluminum is generally used to form wirings and electrodes of semiconductor devices. However, since aluminum is a soft metal, it is not always best suited for high-power press-contact type semiconductor devices. This is because the semiconductor element of this type of semiconductor device is pressed with a higher pressure, since such a semiconductor device is formed to have a higher power. The term "press-contact" used in this specification means that two things can be brought into contact together simply by pressing them against each other, using a high pressure. Assume that the semiconductor element is a power transistor having a diameter of 40 mm. In this case, the emitter and base electrodes of the transistor are formed of aluminum layers of a thickness of, for example, 10 .mu.m. The emitter and base electrodes are alternately arranged at intervals of approx. 200 to 300 .mu.m, with each of the emitter electrodes being formed at a position higher by 20 .mu.m with respect to that of each of the base electrodes. The power transistor is sandwiched between external electrodes of copper and press-contact with the external electrodes under a pressure of 1.0 to 1.5 tons. A heat buffer plate is placed between the emitter and each of the base electrodes of the power transistor and the external electrodes.
Even if the heat buffer plate of the above-described power semiconductor device is inserted between the emitter electrode and the external electrode, the emitter electrode will undergo thermal fatigue after it has been subjected to heat cycle due to repetitive switching operations, for example. Since thermal fatigue may cause the emitter electrode to be put out transversely, with the possibility that it will come into contact with the base electrode, therefore the emitter and base electrodes are set in contact with each other, making part of the semiconductor device defective. As thermal fatigue increases, the heat buffer plate is press-contact with the semiconductor substrate on which the semiconductor element is formed by means of the emitter electrode. If, in this condition, the heat buffer plate is subjected to thermal expansion, this may cause the semiconductor substrate to crack due to stretching.
Cracking of the semiconductor substrate, as well as short circuiting between the emitter and base electrodes, can be prevented by using metal which is as soft as aluminum to form the heat buffer plate. However, when the heat buffer plate is formed of soft metal, there is a possibility that the external electrodes may be deformed when the semiconductor element is press-contact, making it impossible to apply a uniform pressure to the semiconductor element. Thus, it becomes difficult to control the pressure applied to the semiconductor element by means of the external electrodes.
As has been described above, thermal fatigue of the electrode of the prior art press-contact type semiconductor device occurs when the electrode is subjected to heat cycle caused by repetitive switching operations, which causes short-circuiting of the emitter and the base electrodes and cracks in the semiconductor substrate.